1. Field of the Invention
The present invention generally relates to digital telephony. More particularly, the present invention relates to splitting frames of a serial data stream into component fields using a programmable frame splitter.
2. Discussion of the Related Art
Digital telephony data is transmitted, serially, between a PBX (Private Branch Exchange) and a digital phone set. A PBX is a multi-line switching network that is typically located on the premises of a business. The switching network establishes connections between two PBX-supported telephones or between a PBX-supported telephone and an off-premises telephone. PBX/digital phone set transmission schemes are typically proprietary.
A common transmission scheme uses short frames of information on a periodic basis. Within each frame there is a predetermined order of fields. One of the key elements of implementing a telephony protocol stack is the ability to split the frame into component fields. While the problem is easily solved by software using masking and shift operations, the data transmission rate and number of connections that each microprocessor must support makes the solution computationally intensive. As a further complication, telephony equipment is very cost sensitive. Therefore, a microprocessor with the necessary processing power is prohibitively expensive
The network most commonly used to support digital phones is the Integrated Services Digital Network (ISDN). With ISDN, voice and data are carried by bearer channels (B channels) occupying a bandwidth of 64 Kbps (Kilobits per second). A data channel (D channel) handles signaling at 16 Kbps or 64 Kbps, depending on the service type. ISDN includes at least two B-channels carrying user data and a D-channel that primarily carries signaling data. The Basic Rate Interface (BRI) has two B-channels and a single D-channel, possesses a bandwidth of 144 Kbps, and is sometimes referred to as a 2B+D interface to the ISDN.
The signaling capabilities of an ISDN are significantly superior to the current public network supporting analog phones. Typically, key phone systems and other private exchanges are connected to a public switched telephone network (PSTN) to provide voice communication service. In contrast to the conventional telephone network, an ISDN offers a variety of features including multimedia communication service such as voice, high-speed data and image communication services, and other additional non-voice communication services via network interfaces on the basis of digitization of the telephone network.
A PBX transmits to a digital phone set using proprietary protocols that typically follow many of the same principles as the ISDN protocol. Therefore, a discussion of the ISDN data transmission scheme serves as a basis for understanding proprietary PBX/digital phone protocols.
In a U.S., the telephone company provides its BRI customers with a U-interface. The U-interface is a two-wire (single pair) interface from the phone switch. It supports full-duplex data transfer over a single pair of wires, therefore only a single device can be connected to a U-interface. This device is called a Network Termination 1 (NT-1). The NT-1 functions as a frame splitter.
The NT-1 is a relatively simple device that converts the 2-wire U-interface into the 4-wire S/T interface. The S/T interface supports multiple devices (up to 7 devices may be placed on the S/T bus) because, while it is still a full-duplex interface, there is now a pair of wires for receive data, and another pair for transmit data.
Digital telephony data is transmitted to the U-interface. Each U-interface frame is 240 bits long. At the prescribed data rate of 160 kbps, each frame is therefore 1.5 msec long. There are 216 2B+D bits placed in each 1.5-ms basic frame.
The B1- and B2-channels and the D-channel (subrate channels) are multiplexed in a basic frame. A channel occupies an integer number of time slots and is in the same time-slot position in every frame. The B1- and B2-channels and the D-channel may be thought of as component fields. To demultiplex the B1- and B2-channels and the D-channel, it is necessary to split the basic frame into the component fields. For ISDN, this is accomplished by the NT-1 frame splitter.
Frame splitting typically relies on a dedicated hardware implementation to split the frames of the serial data stream into fields. One such implementation is shown in FIG. 1. The inputs into the splitter are: 1) a frame start signal, and 2) a bit clock. The output for each field is an envelope signal that specifies the time of the field. Components that process the field data use 1) a serial data stream, 2) the bit clock, and 3) the field envelope to extract the field's data.
When using a fixed frame splitter, a new hardware design is needed as each protocol stack is implemented. This arrangement results in a proliferation of designs that must be debugged, documented, and maintained. Therefore, there is a need for a programmable frame splitter that does not require a dedicated hardware implementation to split the serial data into component fields.